adding testbenches

gates/testbench/not_tb.v

@@ -0,0 +1,15 @@
+include "not_gate.v"
+
+module not_tb();
+    reg ta;
+    wire ty;
+
+    not_gate dut(y, ta);
+
+    initial begin
+        ta=0;
+        $monitor("IN: %b", ta, "OUT: ", y)
+        ta=1;
+        $monitor("IN: %b", ta, "OUT: ", y)
+    end
+endmodule